Related art to this invention involves a melting device known as a “stack melter”. But stack melters are exclusively heated by combustion burners and most typically fueled by natural gas. They are two chamber devices with the first chamber, or stack, receiving the solid charge to be melted and the second chamber receiving the semi-plastic or partially melted charge to complete the melting process.
A natural gas burner typically supplies heat in the form of high temperature combustion products to the bottom of the stack, with flow counter current to the movement of charge through the stack. The high surface area of the solid charge provides for favorable heat transfer conditions. Accordingly, the charge is typically heated to a state where it begins to flow and pass out of the stack and into the second chamber. The second chamber is heated by a burner with products of combustion passing into the stack to augment heating of the charge. This burner also provides heat to offset thermal/containment losses from the melter. In this sense, the second chamber of a stack melter is similar to a conventional burner-fired gas melter. A stack melter can therefore be characterized as a conventional gas fired melter with a charge preheating stack and regeneration through this charger preheat.
Although heat transfer is favorable in the first chamber or stack portion of the melter, conditions in the second chamber are not. The metal bath has a planar interface and therefore limited surface area for heat transfer. These “bath flat” conditions depend entirely on heat transfer dominated by radiation from the burner flame and surrounding refractory walls. The same inefficiencies inherent in conventional gas melters are present here. Surface heat transfer is typically limited to approximately 115,000 BTU/hr-ft2, and the thermal efficiency in this part of the melter is generally less than 30%. Typically, the amount of metal contained in the second chamber of the stack meter is 5× to 10× the melt rate in lb/hr (i.e.: a 4,500 lb/hr melt rate would require a 22,500-45,000 lb bath).
Preferred Embodiments and General Operation
This melter has two chambers. The first chamber is a shaft that accepts solid charge. A fossil fuel burner produces hot gases that are introduced into the bottom of the shaft. These gases pass counter current to the charge direction and impart heat to the charge. Diluent air may be used to lower the temperature of these gases and avoid excess oxidation of the charge. A grate or other suitable means supports the charge in the shaft until the charge becomes semi-plastic (“mushy”) and no longer can be supported by the grate. Since the charge is heated by a counter current flow of combustion products with or without diluent air, the process is regenerative.
The second chamber consists of a liquid bath of melted charge below the first chamber. Preferably, the liquid bath is circulating within this chamber to improve heat transfer. This second chamber receives semi plastic charge that drops down from the shaft or first chamber.
Additional heat is imparted in the second chamber at high flux to complete the melting process and add sufficient superheat to provide the desired melt discharge temperature. Direct Immersion (DI) electric heaters are preferred for this purpose. The graph at FIG. 4 depicts melting energy apportionment for this melter using a 356 alloy (Al-7% Si) charge. Although commercial 356 alloy typically contains a nominal Mg concentration of 0.35%, the aluminum silicon phase diagram will be used to illustrate the fraction liquid at various temperatures. The aluminum rich side of a simplified Al—Si binary phase diagram appears in FIG. 4. The eutectic composition depicted in that diagram occurs at 12.6% Si and the eutectic reaction isotherm temperature is 570° C. (1071° F.). At that temperature, the maximum solubility of Si in α-Al at the left hand terminal solid solution is 1.65%. It can be seen through an application of the binary lever rule that the eutectic phase represents 48% of an Al-7% Si alloy at 1071° F. by:
An Al-7% Si alloy theoretically requires 511 BTU/lb to melt. Charge material at room temperature (70° F.) is continuously introduced into the top of the first chamber or shaft. Hot air from the first chamber burner begins to preheat this charge.
The total heat (Qt) required to melt the solid charge of mass (M) presented to the melter is the sum of the sensible and transformation heats. The following simple equation applies:
Where: Σ=sum of sensible heats for solid and liquid                Cp=solid and liquid heat capacities, as appropriate        T=temperature, and        ΔHm=transformation heat (heat of melting        
The temperature in the shaft progressively increases toward the bottom since the hot air flows counter-current to charge movement. Charge temperature increases accordingly. The charge will remain in the shaft until it the solidus temperature is reached and the material becomes mushy. For this alloy, the solidus corresponds to the eutectic reaction isotherm temperature of 1071° F. (See, FIG. 5). The total quantity of heat theoretically adsorbed by the charge to this temperature is 260 BTU/lb. All of this heat is sensible and only a function of the charge heat capacity and mass. Hence, it will therefore only increase the temperature of the charge. At 1071° F., the charge will begin adsorbing transformation heat isothermally as the eutectic micro-constituent melts. The eutectic phase comprises 48% of the microstructure and will adsorb an additional 146 BTU/lb until it melts and the charge becomes mushy. Once this mushy condition is achieved, the charge is no longer supportable by the grate at the bottom of the shaft, and the charge falls (drips) into the molten metal bath in the second chamber situated below the shaft. A total of 406 BTU/lb has been absorbed by the charge at this station in the melter. Essentially all of this heat has been supplied by the countercurrent hot air flow entering the bottom of the shaft. Once the mushy charge leaves the shaft or first chamber, no additional opportunity exists for direct heat transfer from the countercurrent hot air flow.
The description and operation of the melter to this point is somewhat similar to a conventional stack melter with the exception that such melters typically use high temperature (>2000° F.) air directly from a burner or from direct flame impingement. These practices result in oxidation of the charge and can limit the gage of charge melted. In the case of this invention, the combustion temperature is reduced by air dilution to approximately 1200° F., while increased forced convection is used to enhance air to charge heat transfer.
Conventional stack melters do not actively control the temperature at which the solid charge enters the bath with the consequence of lost opportunity to add maximum transformation heat and greatest thermal efficiency.
In conventional stack melters, heat supplied the molten metal bath in the second chamber is also derived from a combustion-burner system. Since the bath surface is planar and lacks topographical surface area, a large bath area exposure is required for effective heat transfer. This requirement necessitates a high quantity of contained metal which significantly increases melter size and associated heat losses.
After the mushy charge enters the bath, an additional 105 BTU/lb is added by the molten metal to completely melt the charge and therefore raise its temperature to the liquidus temperature of 1135° F. in this example. Heat transfer is partially isothermal as an additional 52% of the charge consisting largely of primary (α) aluminum, melts. Metal is to be withdrawn from that melter at a temperature of 1300° F. As such, an additional 43 BTU/lb of superheat is required as sensible heat.
The total quantity of heat supplied is 554 BTU/lb with 73% of that heat provided by the first chamber and 27% by the second chamber. In contrast to second chamber heat being derived from a combustion process in conventional stack melters, this invention uses an electric resistance heat source immersed in the molten bath. This heat source uses thermal conduction as the dominant heat transfer mechanism to impart heat to the melt and forced convection to transfer heat throughout the melt.
Since the heat source(s) is/are immersed, heat transfer occurs in the absence of air and is volumetric, i.e.: independent of melt surface area.
In this invention, conditions that determine when the mushy charge enters the liquid bath are influenced by both design parameters and operating parameters. One embodiment of the invention supports the charge using refractory posts that project up from the melt below the preheat chamber. A quantity of solid charge (charge column) is maintained in the preheat chamber and imposes a load over portions of the charge that bridge the refractory posts. This creates a 3-point loading condition where the reaction forces are provided by the posts, and the weight of the charge applies a load to the portion of the charge spanning between the posts. Based on the range of alloys to be melted, the spacing between the posts determines the magnitude of the applied bending stress. The particular charge form for this example can be approximated with cylinders since the charge consists of gate and sprue (revert). Accordingly, the section modulus and moment of inertia corresponding to cylinders are used in design. The point at which the charge column can no longer be supported by the span between the posts is determined by the maximum local bending stress and rupture strength of the material being melted. Rupture strength is determined by the material properties at the local temperature.
A series of experiments were conducted with 319 alloy (nominally 6% Si, 3.5% Cu, balance Al) to determine rupture strength between the solidus and liquidus temperatures, i.e., 960° and 1120° F., respectively. These experiments subjected a 0.5 inch diameter solid cylinder to 3-point loading over a 2.0 inch span at several temperatures. Each data point represents the stress at failure, as determined by textbook relationships between applied load maximum bending moment, and the moment of inertia for a solid cylinder.
It can be seen from the summary graph at FIG. 6 that 319 alloy will rupture at an arbitrary 100 lb/in2 load and 1035° F. Since the delivered exhaust air temperature from the turbine to the charge preheat chamber is almost 1100° F., designing the charge column support post spacing to impart a 100 lb/in2 stress magnitude is reasonable as a design parameter.
Minor alloy chemistry variations, ambient temperature variability, and changing loading conditions on the turbine will impact on preheat chamber air temperature. A means is therefore provided to introduce dilution air for the purpose of manipulating local charge temperature to optimize the drop in point to the liquid bath. Preferably, non-contacting optical pyrometry is used to measure charge temperature, but conventional thermocouples imbedded in the charge preheat chamber wall can be used for this purpose. Closed loop control is used to regulate air temperature to result in optimized charge preheat chamber operation. In situations where alloy chemistry and corresponding solidus/liquidus temperatures require an air temperature higher than delivered by the turbine, an air dilution afterburner can be used. One such burner is a Model 4422 high pressure burner available from Fives North American Combustion, Inc. that can add a small quantity of heat to increase air temperature.
Potential benefits of this invention include:
1. Substantially higher heat flux based on bath surface area. Typical conventional burner heating operates at 125,000 BTU/hr-ft2 bath area burner head to recover 31,200 BTU/hr-ft2 in the melt. The invention heating methodology provides a net 337,000 BTU/hr to the same bath area. The result is more efficient heating and reduced metal containment. Furthermore, the invention can be less than 25% of the size of a conventional stack melter making it comparatively small and portable.
2. A lower holding energy for metal contained in the second chamber.
3. A sub-surface/anaerobic heat transfer with no associated oxidation.
4. An ability to respond faster to changes in charge rates.
5. Since no burner is used in second chamber, melt surface is not superheated. That causes some vaporization of the melt with consequential melt loss and downstream fouling in the stack due to condensed metal vapor and oxides.
6. Refractory life should be improved due to lower metal line and above metal line wall temperature.
7. Melting costs should be reduced due to higher thermal efficiency and reduced holding volume. Based on $4.00/dth natural gas, the energy cost only for a conventional 5,000 lb/hr stack melter is about $0.0056/lb. By contrast, the invention will melt at about $0.0045/lb (See, the highlighted “Stack ITM” sections in the accompanying chart).
8. The invention has the capability of adjusting fossil fuel/electricity ratios based on energy costs. Such ratios can be manipulated based on energy cost differentials.
9. Various sources of electricity can be used with this invention. One embodiment combines the melter with a gas turbine powered generator with exhaust gas enthalpy recovered in the shaft. (Note, the “CoGen ITM” reference in the chart below.)
10. The invention is not dependent on externally applied electricity.
11. It is capable of producing electricity when not being used for melting.
12. It can also operate on lean (i.e., low BTU) and green (landfill gas) fuels with a front end gas conditioning train.